Transmission line termination



Oct. 13, 1953 O w o w D, JR 2,655,635

TRANSMISSION LINE TERMINATION 7 Filed Dec. 21, 1948 l I a) I 1 w l I .8 I l T l W5 1 a z I 524 326 325 550 aaz 554 356 338. 540

INVENTOR Oaklg jll wooa'wardzln jfhm ATTORNEY Patented Oct. 13, 1953 Y 2,655,635 TRANSMISSION LINE TERMINATION Oakley M. Woodward Jr., Princeton, N. J., as-

signor to Radio Corporation of America, a corporation of Delaware Application December 21, 1948, Serial No. 66,497

9 Claims.

This invention relates to terminations for transmission lines and particularly to a tunable high frequency transmission line matched resistive termination.

It is frequently desirable to terminate a high frequency transmission line, for example, a coaxial transmission line with a substantially perfectly matching termination which will absorb substantially all of the energy transmitted through the line to the termination without the formation of reflections or standing waves. Such transmission line terminations are frequently difficult to construct, and when constructed, are often suitable for use at only one frequency. Furthermore, some of these terminations are lon and consequently awkward to handle, and comparatively expensive to construct, with a special termination being required at each frequency employed.

It is an object of the present invention to improve the termination of a high frequency transmission line.

Still another object of the invention is the provision of a high frequency transmission termination which will be suitable over a Wide range of frequencies.

Another object of the invention is to provide a high frequency transmission lin termination which shall be compact, inexpensive, and readily constructed.

These and other objects, advantages and novel features of the invention will be more fully apparent from the following description taken in connection with the accompanying drawing in which:

Fig. 1 is a transverse cross-sectional view of a transmission line termination in accordance with the invention; and

Fig. 2 is a plot showing the high degree of perfect matching and absorption possible with the termination of Fig. 1.

In accordance with the invention, a first length of coaxial transmission line is arranged and adapted to be connected to the coaxial transmission line to be terminated. The first length of line includes a lossy material between the conductors thereof to absorb energy from the electromagnetic field between the conductors, and a capacitive element such as a projection or protuberance is provided between the line conductors which, in effect, provides a lumped capacitance at a chosen point along the line. By suitably positioning th lumped capacitance, an impedance is 2 reflected at the connecting end of this length of transmission line having a resistive component which is equal to the impedance of the transmission line to be matched. A second transmission line is inserted in series with the first length of transmission line and which is substantially entirely reactive and of opposite sign to cancel the reactive component of the first length of transmission line. The line is thus substantially perfectly matched by the terminationat any desired frequency over a relatively wide range.

Referring now to Fig. 1, an adapter H] ha an outer threaded conductor l2 and an inner tapered conductor l4. Thus means or terminals are provided for connection to the line to be matched. The adapter I 0 illustrated is designed for easy connection to a standard 52 ohm characteristic impedance coaxial line. Conductor I2 is connected to a cylindrical outer conductor [6 terminating in a short-circuiting portion l8 which short-circuits conductor I 6 to an inner conductor in the form of a sleeve 20 surrounded by the conductor I6. Sleeve 20 surrounds conductor 22 which is an extension of conductor l4. Sleeve .20 and conductor 22 form a coaxial line short-circuited by a shorting plug 24 longitudinally adjustable therewith. Between conductors l6 and 20 is a capacitive projection or protuberance, in this instance a ring 26 of conductive or dielectric material, as desired, connected to conductor IS. The ring may be moved longitudinally (as indicated) along conductor It by a longitudinal slot therein (not shown) or by removing end plate I3 from its connection with conductor l6 by taking out screws 28, and then replacing the parts. Short-circuiting plug 24 may also be slid longitudinally (as indicated) of the structure and may be reached through the inactive open end 30 of sleeve 20 at the back of the structure. Conductor 20 is surrounded by Or has a heavy coating of lossy or resistive material 32 distributed along the length of line section I6, 20, as shown. In fact, the structure may be formed by taking an ordinary conventional 50 watt resistor, of tubular form, such as are in common commercial manufacture, and inserting the tubular sleeve 20 within it. The particular termination of Fig. 1 is about eight inches long overall, and about 1% inches in outside diameter.

In operation, the adapter I0 is inserted into a mating connection in the coaxial line. The ring 25 is moved to a suitable position, at which point there is reflected at th adapter end ill a heavily resistive component, due to the resistive material 32 in the electromagnetic field between conductors l6 and 20, in shunt with the capacity introduced by the ring 26. The amount of resistance thus reflected at the adapter I may be controlled by positioning of the ring 26. There will then be presented at adaptor an impedance comprising a resistive component and a reactive component. The short-circuited length of m ssi l e orm by ond ctors 0. a d 22 is eff "tively in series with this reflected ime pedance etween the adapter terminals. By suitable adjustment of the shorting plug 24, the reactive component of the reflected impedance may be tuned out or balanced out by areactance of the same magnitude but opposite in sign.

As well understood, the resistance of the ma: terial 32 will change with frequency. However, it is a Simple matter to readjust the termination device to properly terminate, the line at the new: operating frequency.

Referring now more particularly to Fig. 2 there is shown a curve of standing wave ratio plotted against frequency for a standard 52 ohm coaxial transmission line terminated with the device of- Fig. 1. This curve was derived experimentally for! an adjustment ofthe shorting plug 24 and ring 26 of the termination of Fig. 1 for a proper termination at 333 megacycles. Similar curves may be derived with a unity standing wave ratio at other frequencies by readjustment ofthe plug and ring. It will be noted that the resistive or lossy material employed maybe a thick coating, and is not necessarily a thin or surface coating. Therefore, I may use standard resistors to make the line, instead" ofhaving the difiiculties associated with preparingsprays and the like for forming a lossy material surface coating on conductor 20, as is necessary in some prior matching terminations. Moreover, no complicated logarithmically curved surfaces need be formed, as in some prior art terminations.

It will'be apparent that I; have provideda transmission line termination which may be readily and suitably adjusted for different ranges of high e ency d w n v s mp conom a No separate or new construction is necessary to arrange the. device to match a desired transmission "ne at different high, frequencies, only a .1 2 and, read ma e i s m m, we. 9; ha n ve What I l ims;

a termina ion r a fi s rew m s n e ompr si first s ree a ng nn r an ne: conductors terminated for. connection tosaid line, asecond; portion having an innermost conductorcontinned from said first portion inner. conductor: 3 ii l 5. rr i id second P0111 tion innermost conductor and short-.circuited he et a a Poin distant from id rs P r on, aid second portion ha n an ou rm t we: seem: est nd d, o d. ir t rt n c t r: 00. 1 dugtor, said sleeve beingsurrounded by outermost conductor and short-.circuited theretoat a Pa t d s a from ai rst porti n cond martian thereby m r in e a fii shqrtrgrquited 1, 1 u mission. lines each with inner and outer OOIP.

d mers h a sleeve being h Q i t nque or;

of said inner line and the inner conductopofsmd u r .19- Qn .v a h tir tedir nsm sen in a n an me t n ap c t v tipnship between the conductors thereof and hav- 1 'a q y at r di t iba 'a qng the length 4 of said one transmission line between the conductors thereof, said short-circuited lines hav n cancelling reactances.

2. The termination claimed in claim 1, said lossy material being a resistance coating on one of the conductors of said one short-circuited transmission line.

3. The termination claimed in claim 1, said element being metallic and ring-shaped and connes ed di e tl on onduc or of said on shortcircnited line.

4. The termination claimed in claim 1, said element being adjustable in position along said one short-circuited transmission line.

5. The termination claimed in claim 1, said one shorteci icuited transmission line being the outer one, the shortecircuit of the other inner transmission line bei igg adjustable whereby said other inner short-circuited transmission line is adjustable in length.

6 termination for a first two conductor transmission line, said termination comprising an innermost conductor. an intermediate conductor surrounding said innermost conductor, and an outermost conductor surrounding said intermediate conductor to form an inner transmission line section and a outer transmission line section, saidintermediate conductor serving as both the outer conductor of said inner line section and the inner conductor of saidouter line section, means to connect one end of said innermost conductor to one conductor of said first line, means to connect said outermost conductor at its one end ad-. jacent said innermost conductor one end to the other conductor of said first line, said innermost conductor at a point remote fromits said one end being short-circuited to said intermediat conductor, said; outermost conductor at a point re mote from its said on end being short-circuited to said intermediate conductor, the said line sections being thereby serially connected, one of; said line sections having a lossy material distributed along the length of the section between the conductors thereof, said one line section having a capacity element, between its conductors, said line sections having cancelling reactances. thereby to present a purely resistive termination to said' firstline on connection thereto.

7. The termination claimed in claim 6, saidlipe section conductors being coaxial.

8:. h mi iqac a m n sa dliee. section conductors being coaxial, said capacity element being a ring shaped capacity projection from oneofthe conductors of said one section 9. A termination for a first two conductor transmission line, the termination comprising three conductors, the second being positioned between the first and third thereby to form two transmission line sections of which said second conductor is a conductor for each, means to con; nect said first conductor at one end thereof to e on tor. s id. i st an mis on. e. means to connect the one end of said third eonductor adja t. aid ne a d 1' sa d. irst oni c o. the o e qn ctor. qf sa d r t t an mission line, said second conductor being short,- ir uit dt said firs t a Po n r m e f om aid,

.first cor tiuctxqr one end, said second conductor ige sh t r uit q o sa d h r cqnsi c at a po nt. mot rom a d thi d c d or enowher Said. ne s c on r er ly .0853- me e ie w en.- hose w c nductors forming one of line section effective to. ab,-. e b. w re! in h e ee r asne fields e enmaiee el e ne d t but l ne the References Cited in the file of this patent UNITED STATES PATENTS Name Date Espley Oct. 8, 1946 Number Number 6 Name Date Tiley Oct. 15, 1946 Gunter Jan. 13, 1948 Hansen Apr. 6, 1948 Webber Mar. 15, 1949 Wheeler Oct. 3, 1950 OTHER REFERENCES The Series Reactance in Coaxial Lines, by H. 10 J. Rowland, from Proceedings of the I. R. E., January 1948, vol. 36, No. 1 found on pages 65-69. 

